CHAPTER XXI 



AIR AND WATER COMPARED. ATOMS AND 

 ELEMENTS 



You have learned that water, like all other substances, 

 is composed of molecules. You have noted that a molecule 

 of water is composed of hydrogen and oxygen, the chemical 

 symbol indicating this composition being H 2 O. 



Now as to air. You may ask, is there such a thing as 

 an air molecule, and if so, what is its composition ? Already 

 you know that air contains oxygen, which we use in breath- 

 ing, and nitrogen, and a little carbon dioxide. So we ask, 

 are these substances parts of air molecules, as hydrogen 

 and oxygen are parts of water molecules, or do they exist 

 in some other relationship ? 



The fact is that there is no such thing as an air molecule. 

 Pure water is composed of just one kind of molecule, but 

 pure air is composed of a number of different kinds. Water 

 is what is called a compound, while air is a mixture. 



Here we have come upon a very important fact. If we 

 stop to consider it carefully, we shall get some important 

 information as to how all substances are composed. Water 

 a compound, and air a mixture ! The thing for us to do is 

 to see just what is meant by this word compound, and 

 then to see why it does not apply to air as well as to water. 



Molecules. To do this we must look a little further 

 into the nature of molecules. A molecule is the least 

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